Microbeam Radiation Therapy (MRT) is a novel approach that utilizes parallel, thin (<100 mum wide) planes of synchrotron-generated x-rays (microplanar beams, or microbeams). Single-fraction unidirectional MRT preferentially kills gliomas in experimental animals while sparing normal brain tissue, thus producing a higher therapeutic index (ratio of maximum dose tolerated by normal tissue to the minimum dose ablating the tumor) than conventional broad-beam therapy. We hypothesize that the normal-tissue-sparing of MRT is due to the replacement of lethally injured endothelial cells in the direct paths of the microbeams by their neighboring cells surviving between the beams. This proposal will evaluate the above hypothesis, and examine possible enhancement of the MRT's beneficial effects by a) dose-fractionation, and b) by gadolinium administration. Intracranial CNS-1 rat glioma and the normal rat brain will be used as models. Microbeams' therapeutic index will be compared to that of broad beams. A major consideration in the experimental design of Aim 1, dose-fractionated MRT, is that MRT dose must be fractionated at intervals, 1-3 weeks, long enough to permit (presumably) vascular repair; this limitation exists because the microbeam pattern from the second irradiation may fall between the pattern from the first one, thus interfering with the repair process. Therefore, mechanistic studies to estimate the recovery time of normal brain tissue from MRT will be pursued first to estimate the recovery time. Unidirectional unfractionated (two equal dose fractions, each tentatively half the unfractionated dose) and fractionated MRT, as well as unfractionated broad beams will be studied. The endpoints will include normal-tissue damage and tumor control assessed by MRI, behavioral tests, survival, and histology. Aim 2, Gd-enhance MRT, is motivated by a) the large uptake of Gd by the glioma after injecting the MRI contrast agent, Gadovist, and, b) Monte Carlo simulations of MRT dose distribution demonstrating that the tumor's the radiation leakage between microbeams (the "valley" dose) in the tumor increases three-fold after Gd uptake. Experiments will include unidirectional microbeam and broad-beam irradiations 5 minutes after Gd administration. Optimizing the highly innovative method of MRT from our findings may significantly boost its therapeutic index, leading to a breakthrough in the radiation therapy of brain tumors.